Process and installation for the treatment of effluents by oxidation in the presence of a heterogeneous catalyst

ABSTRACT

A process of aqueous phase oxidation of effluents, consisting of subjecting the effluents to oxidation in the presence of at least one oxidizing agent inside a reactor having a gaseous phase set up above a liquid phase consisting of the effluents, and subjecting the gaseous phase to catalysis in the presence of at least one heterogeneous catalyst. The oxidation process is carried out at a temperature of between approximately 20° C. and approximately 350° C. under a pressure of between approximately 1 and 160 bars. At least a part of the organic matter and total ammoniated nitrogen contained in the effluents are mineralized. The process includes recycling at least a part of the gaseous phase present in the oxidation reactor after the gaseous phase has passed through the heterogeneous catalyst so as to effectively increase the contact time between the gaseous phase and the heterogeneous catalyst in order to obtain substantial removal of NH 3 , COR, and volatile organic compounds.

The area of the invention is the treatment of industrial or urbaneffluents containing organic matter and nitrogen compounds.

More generally the invention relates to the treatment of effluents whichcontain organic matter and organic and inorganic compounds of nitrogen,such as waste lixiviation products, farm excrements, chemical industryeffluents (dyes, explosives, anilines, nicotinic acid, polyamides etc.)effluents of agro-food industries, treatment plant sludge, outputeffluents from treatment sludge packaging and dehydration etc..

The treatment concerned consists of removing from the effluents to betreated a substantial part of the undesired compounds they contain sothat they can be discharged into a natural receiving environment, atreatment facility or a collector network. The effluent considered maybe water or any other fluid liquid.

The methods conventionally used to treat urban or industrial effluentsuse biological processes intended to reduce their biological oxygenrequirements (BOR) and their content of nitrogenous nutrients andphosphorus. However, certain effluents containing pollutants that arenot easily biodegradable and have high ammonia contents require the useof special processes and/or necessitate the additional use of chemicalsubstrates for their treatment.

One effective treatment adapted to the elimination of chemical oxygenrequirements (COR) is aqueous phase oxidation which has been describedat length in the prior art. The objective of this technique is to carryout extended oxidation of organic matter that is little biodegradablecontained in aqueous effluents through the contact of said effluentswith an oxidising agent. For this purpose the operating conditions ofsaid process typically lie between approximately 20 and approximately350° C. regarding temperature and between approximately 1 andapproximately 160 bars in respect of pressure.

Aqueous phase oxidation processes do not allow substantial eliminationof ammoniated nitrogen, particularly when the effluents to be treatedcontain high concentrations of ammoniated nitrogen (>200 mg/l). Evenoxidation under wet conditions (Wet Air Oxidation) which is one of thebest performing oxidation processes, generally carried out at atemperature of between approximately 200° C. and approximately 350° C.and a pressure generally lying between approximately 20 andapproximately 160 bars, only achieves limited removal of ammoniatednitrogen with yields of 5% to 10% whereas organic carbon is destroyedwith efficacy in the region of almost 80%. Numerous publications haveshown that the treatment of industrial or urban effluents by wet airoxidation only achieves very partial elimination of total Kjeldahlnitrogen of between 5 and 15% and that on completion of treatment thelatter is essentially in ammoniated nitrogen form.

Physical processes also exist for the removal of ammoniated nitrogen.Air or steam stripping, effective for high contents, requiresconsiderable investment and is ill-adapted to the treatment of effluentswhich also contain high concentrations of organic matter. Also, it onlyachieves ammonia conversion by concentrating it. With this type ofprocess the ammonia is removed by neutralisation with sulphuric acid toform ammonium sulphate which has to be stored before being put tofurther use, which constitutes an additional operating charge. Withtreatment by wet air oxidation for example this operation can only becarried out after leaving the effluent to settle, cooling to atemperature of less than 80° C. and adjusting pH in order to preventsimultaneous release of volatile, foul smelling and/or harmful organiccompounds during forced aeration at a higher temperature. This treatmentof ammoniated nitrogen subsequent to wet air oxidation leads to muchincreased investment and operating costs.

In treatment plants the removal of ammoniated nitrogen may also be madeby biological nitrification-denitrification treatment. This treatmentdoes not easily accept high loads.

If the effluent has sufficiently high COR content it is possible tocarry out simultaneous removal of organic matter and of organic andinorganic nitrogen compounds by incineration. This technique leads tothe formation however of a large quantity of NOx nitrogen oxides (x=1and 2), by oxidation of a substantial part of the nitrogenous load. Inorder to comply with NOx release standards therefore, it is necessary totreat incineration fumes, in particular by catalytic reduction of NOx byNH3, a technique which is expensive to set in operation.

It is also possible to reinforce the efficacy of wet air oxidation forthe removal of ammoniated nitrogen through the use of heterogeneouscatalysts in contact with the effluent to be treated, made up forexample of titanium dioxide, a rare earth oxide and a precious metaloxide such as described in European Patent EP-A43 1 932, or in AmericanPatent U.S. Pat. No. 3,988,259. However, such catalysts have thedisadvantage of showing substantial loss of activity with time due tothe fact that they are immersed during use. A further disadvantage ofcatalytic wet air oxidation arises from the fact that the heterogeneouscatalyst may be affected by the precipitation in its structure ofcalcium carbonates and sulphates and of metals present in traces in theeffluents such as mercury, cadmium, lead, zinc etc. which are knownpoisons for numerous catalysts by acting to form combinations or alloysin particular with precious metals. All these disadvantages mean thatthe process of wet air oxidation is not currently used to treateffluents.

It will also be noted that as no catalysts are used for processes suchas wet air oxidation for example, this leads to gaseous ammonia beingcarried by treatment gases which causes the formation of ammonium saltdeposits such as ammonium sulphate, ammonium acetate etc. These depositsmay lead to fouling of essential parts such as conduits and valves.

The purpose of this invention is to provide a process for the oxidationof effluents in aqueous phase which will remedy the disadvantages of thecurrent state of the art. More precisely, the purpose of the presentinvention is to provide a process for treating industrial or urbaneffluents containing organic matter and organic and inorganic nitrogencompounds which achieves substantial removal of total ammoniatednitrogen and simultaneously achieves a substantial decrease in the CORof said effluents and in the release of harmful or foul smelling gases.

A further objective of the invention is to provide a process andinstallation which allows disadvantage-free use of heterogeneouscatalysts for wet air oxidation processes.

Yet another objective of this invention is to describe a process whichwill substantially increase the life of such heterogeneous catalysts.

A further objective of the invention is to improve the efficacy ofaqueous phase oxidation processes and to reduce the costs incurred fortheir implementation.

Whereas French patent application n° 9413100 of Oct. 27th 1994 (filedbefore the anteriority date of the present international application butonly published after this date,) recommended, in order to achieve theseobjectives, placing the heterogeneous catalyst actually inside theaqueous phase oxidation reactor above the interface between the gasphase and the liquid phase, subsequent research work highlighted thatthese different objectives could be achieved whether the catalyst wasplaced inside the reactor or outside the reactor on the outlet pipe fordischarging the gaseous phase but making provision for a recycling stageof at least part of the gaseous phase present in the oxidation reactor.

The invention claimed in the present application therefore relates to aprocess of aqueous phase oxidation of effluents consisting of subjectingsaid effluents to oxidation in the presence of at least one oxidisingagent inside a reactor in which a gaseous phase is set up above theliquid phase made up of the effluents, and of subjecting said gaseousphase to catalysis in the presence of at least one heterogeneouscatalyst, said process being carried out at a temperature of betweenapproximately 20° C. and approximately 350° C. under a total pressure ofbetween approximately 1 and approximately 160 bars, in such manner as tomineralise at least part of the organic matter and total ammoniatednitrogen contained in said effluents characterized in that it comprisesa stage consisting of recycling at least part of said gaseous phasepresent in said oxidation reactor after transiting through saidheterogeneous catalyst, in such manner as to ensure sufficient contacttime with said effluents in order to obtain substantial removal of NH3,COR and volatile organic compounds.

It will be understood that according to the new aspect of the invention,the catalyst may be placed outside the core of the reactor on therecycling pipe of said gaseous phase.

However, according to a variant of interest of this invention, saidheterogeneous catalyst is placed inside said reactor above the interfacebetween said gaseous phase and said liquid phase, as described andclaimed in French patent application n° 9413100.

With said process it is possible to achieve removal of total ammoniatednitrogen by oxidation into N₂ molecular nitrogen without forming NOxnitrogen oxides(x=1 or 2).

The catalyst used in this way is also able to achieve simultaneousremoval of the carbon monoxide CO usually formed during wet airoxidation through oxidation into carbon dioxide, and the removal ofvolatile organic compounds by oxidation into carbon dioxide and water.

It was found, in surprising manner, that such positioning of thecatalysts inside the reactor allowed the removal with great efficacy ofboth ammoniated nitrogen and CO which in turn allowed release of theresidual gas into the atmosphere with no complex subsequent treatment.In unexpected manner, the transfer of ammoniated nitrogen to the gaseousphase of the reactor, as far as the catalysts in view of oxidation, issufficiently efficient to avoid having to proceed with pH adjustment tohigher levels as is the case with forced aeration.

The position of the heterogeneous catalyst above the interface betweenthe gaseous and liquid phases in the oxidation reactor also avoids theuse of costly catalysts able to resist against the corrosive conditionsof the liquid phase, and also avoids any risk of particle fouling of thecatalyst and any risk of loss of activity of the catalysts bydissolution of its active phase or by reaction with contaminants presentin the liquid phase.

According to a variant of interest of this invention, the process is setin operation at a temperature of between approximately 200° C. and 350°C. under a total pressure of between approximately 20 and approximately160 bars. It therefore constitutes a process of wet air oxidation.

Preferably, said heterogeneous catalyst is a metal belonging to thegroup made up of vanadium, niobium, chromium, molybdenum, tungsten,manganese, iron, cobalt, nickel, copper, cerium, platinum, rhodium,palladium, ruthenium and iridium and the mixtures and compounds of oneor more of these.

The catalyst may advantageously be placed on a mineral support made upfor example of an oxide such as alumina, silica, zeolites, titaniumdioxide, zirconium etc.

The catalysts may be prepared by any other means known to men of theart, in particular by impregnation of a porous support with a solutionof one or more compounds of metals producing metals or metallic oxidesby heat activation, or by a mixture of an oxide support and one or moremetal compounds then given form by extrusion, pelleting, granulation,compressing etc.

The catalyst of the invention may be in the form of beads, chips,cylindrical or polylobate extrudates, rings, ceramic or metallichoneycombs, or any other form appropriate for setting up a fixedcatalyst bed placed in the wet air oxidation reactor above the interfacebetween the gaseous and liquid phases. Preferably, metallic honeycombsare used as they have the combined advantages of being cheap, easy touse, easy to lock into position inside the reactor and easy to moveinside the reactor.

As specified above, oxidation in aqueous phase is carried out in areactor operating continuously or intermittently at a temperature ofbetween approximately 20° C. and approximately 350° C. under a totalpressure lying between approximately 1 bar and 160 bars. To perform saidoxidation at least one oxidising agent is used chosen from among air,oxygen enriched air, oxygen, ozone, hydrogen peroxide, peracids, gaseouschlorine, chlorine bioxide, sodium hypochlorite, potassium permanganateor any other oxidising agent known to men of the art.

If the oxidising agent used is placed in the treatment reactor in liquidor solution form, as for example hydrogen peroxide, sodium hypochlorite,potassium permanganate etc . . . the invention preferably comprises agas flow into the reactor made up of at least one agent chosen fromamong air, oxygen enriched air, oxygen, ozone, water or nitrogen steam.

Catalytic oxidation is carried out at a temperature of betweenapproximately 200° C. and approximately 350° C., preferably between 250°C. and 300° C. When setting in operation a process of wet air oxidation,positioning of the catalyst inside the reactor, owing to the temperatureprevailing inside said wet air oxidation reactor (between 200° C. and350° C.) proves to be highly effective in carrying out oxidationreactions of NH₃ into N₂ and N₂ 0, of CO into CO₂ and of volatileorganic compounds into CO₂ and H₂ 0 without the need to heat the gasesas in the case of treating said gases in an additional reactor locatedoutside the wet air oxidation reactor. Also, since the differentoxidation reactions catalysed in this way are highly exothermic, theheat emitted by said reactions is for the most part transmitted byradiation, conduction and convection to the entire reactor whichimproves its thermal output and in particular enables treatment of morediluted effluents containing less COR without the need to supplyadditional calories to balance the overall thermal output of theoxidation process. according to a variant of interest of this invention,this catalytic oxidation temperature may be higher than the oxidationtemperature in aqueous phase. It will then be possible not to bring theentire inside of the reactor up to catalytic oxidation temperature butonly the area in which this catalytic oxidation takes place, which meansthat lower pressures can be used for oxidation in aqueous phase. To setin operation said variant of the invention, specific heating means areused to heat the catalytic oxidation area, which are placed at the samelevel as the area of the reactor in which the heterogeneous catalyst ispositioned. These means may in particular be made up of a heating collarplaced on the outside surface of the reactor. The catalytic oxidationarea may also be heated using the Joule effect. Heating the catalyticoxidation area to a temperature that is higher than that of the liquideffluent also has the advantage of avoiding any condensation of saideffluent.

According to a variant of the process, said oxidation in aqueous phasemay be carried out in the presence of a homogeneous catalyst intended toincrease the efficacy of COR reduction. According to said variant,oxidation is therefore carried out in the presence of two catalysts, aheterogeneous catalyst placed above the interface between the gaseousphase and the liquid phase, and a homogeneous catalyst.

Preferably, said catalyst is a metal belonging to the group made up ofmanganese, iron, cobalt, nickel, copper, zinc and the mixtures andcompounds of one or more of these. In particularly advantageous manner,a soluble compound of copper is used (such as copper sulphate) or ofzinc or their mixture, the mass ratio of catalyst metal/chemical oxygenrequirements (COR) of the effluent before treatment lying preferablybetween approximately 5.10⁻⁴ and 3.10⁻¹.

It will also be noted that another catalyst may be used at the exit ofthe reactor, for example for additional treatment of the carbon oxideand of volatile organic compounds.

According to a further variant of the process, the treated effluentcomprises an additional stage of recycling part of said liquid phasepresent in the oxidation reactor. With said stage it is possible tofurther increase the contact time between the liquid phase and the gasphase to allow oxidation of the organic matter in said effluent.

Also according to a variant of interest of this invention, the processcomprises a stage consisting of adjusting the pH of said effluents to avalue of 7 to 12. It was observed that said adjustment increased theefficacy of catalytic oxidation of ammonia without increasing theformation of nitrogen oxides.

The invention relates to any reactor for aqueous phase oxidation of aliquid effluent by an oxidising agent in order to set in operation theabove-described process, in which a gaseous phase is set up above theliquid phase made up of the effluents, characterized in that itcomprises means of recycling said gaseous phase.

Preferably said reactor also includes means of holding a heterogeneouscatalyst above the surface of said liquid effluent.

Also preferably, said reactor includes means adjusting the position ofsaid holding means, in such manner as to be able to adjust the heightbetween the catalyst and the surface of the liquid effluent inside thereactor. This height may vary in relation to the type of effluent to betreated, particularly in relation to whether or not stirring means areprovided within the reactor.

According to a variant the reactor comprises a devesiculator between theliquid phase and the catalyst.

According to a variant of interest, the reactor also comprises means ofrecycling the liquid phase.

Preferably said means of recycling the gas phase includes means ofaspirating this gaseous phase after its passage on the catalyst and ofmixing it with the recycled liquid phase.

Such means may, for example, be made up of a hydro-ejector. The use ofsaid means will increase ammonia stripping and therefore oxidation bycatalysis. Also, it provides for better use of the injected oxygen,since the recirculated gaseous phase still contains a high quantity ofoxygen. Therefore, such means allows mprovement of the gas/liquidtransfer and therefore an increase in the efficacy of the oxidationreaction.

As will be explained in more detail in the examples of embodimentdescribed below, it is particularly advantageous to use saidhydroejector while continually adjusting the pH of the treated effluent.It was observed that such treatment achieved a decrease in the ammoiatednitrogen content of the treated effluent. This constitutes a furtheradvantage of the process of the invention since Wet Air Oxidationprocesses traditionally have the disadvantage of producing treatedeffluent having an increased ammoniated nitrogen content.

The invention and the different advantages it offers will more clearlyunderstood on reading the description of the five examples of embodiment(examples 2 to 6) given below with reference to a comparative examplewhich does not use the characteristics of the invention (example 1) andwith reference to the drawings in which:

FIG. 1 represents the formation and reduction of ammoniated nitrogen inrelation to treatment time, to the presence of a homogeneous catalyst(Cu) and a heterogeneous catalyst (Pt catalyst) at 235° C. and 38 bars:

FIG. 2 represents COR reduction in relation to time and to the presenceof a homogeneous catalyst (copper) and a heterogeneous catalyst(platinum);

FIG. 3 represents the influence of the final pH of the treated effluenton the percentage of removal of ammoniated nitrogen;

FIG. 4 represents a first embodiment of a reactor in accordance withpatent application n° 9413100 which does not include means of recyclingthe gaseous phase;

FIG. 5 represents a second embodiment of a reactor in accordance withthe patent application No. 9413100 which does not include means ofrecycling the gaseous phase;

FIG. 6 represents an embodiment of a reactor in acordance with thepresent patent application integrating the means of recycling thegaseous phase, and;

FIG. 7 represents a further embodiment of a reactor in accordance withthe present patent application also integrating the means of recyclingthe gaseous phase.

EXAMPLE 1 Not Representative of the Invention

In a first series of tests, wet air oxidation is examined of a liquideffluent having the following characteristics:

COR: 34.6 g/l

N-NH4 content: 1,89 g/l

pH: 5.41

This effluent is placed in an autoclave reactor in the presence of anoxygen/COR stoichiometry of 1.5, at a temperature of 235° C. and under atotal pressure of 46 bars with a reaction time of 10 min. For comparisonwith a test without heterogeneous catalyst, catalysts containingprecious metals are placed inside the autoclave on an alumina support incylindrical drop form (2.8 mm×3.5 mm) comprising respectively 0.5%ruthenium (615 mg of type 146 catalyst produced by Johnson Matthey),0.5% platinum (610 mg of type 73 catalyst produced by Johnson Matthey)and 5% palladium (110 mg of type 49 catalyst produced by JohnsonMatthey).

The following COR and N-NH4 values were noted at the end of the test.

    ______________________________________                                                 Without catalyst                                                                        0.5% Ru  0.5% Pt 5% Pd                                     ______________________________________                                        COR (g/l)  31.9        31.0     28.0  30.3                                      COR red. (%) 7.8 10.4 19.0 12.4                                               N-NH4 (g/l) 2.28 2.10 1.67 2.33                                               N-NH4 red (%) -17.3 -11.3 -11.3 -23.6                                       ______________________________________                                    

It is observed that the presence of Ru and Pd based catalysts does notsignificantly alter reductions of COR and ammoniated nitrogen. On theother hand, the Pt based catalyst leads to a COR reduction of 19% andremoval by oxidation of 11% of ammoniated nitrogen. However, after areaction time of 10 min, all the catalysts used lost most of theirprecious metal content through suspension in the solution further toshock and friction due to stirring of the effluent in the reactorrequired for reaction purposes. Although it shows some efficacy inremoving ammonia, the platinum based heterogeneous catalyst placed inthe liquid effluent to be treated does not show sufficient long-lastingproperties for industrial use.

EXAMPLE 2

In a second series of tests wet air oxidation of sludge from a treatmentplant with the following characteristics was examined:

matter in suspension : 40.7 g/l

volatile matter: 60.7%

COR: 48.7 g/l

N-NH4 content: 0.938 g/l

pH: 6.3

This sludge is placed in a wet air oxidation reactor according to thepresent invention as shown in FIG. 4.

The reactor is supplied with effluent to be treated by injection pipe1a. This reactor is fitted with heating means able to bring the effluentto a temperature lying between approximately 100° C. and 350° C.Pressurising means are provided to bring the effluents to be treated inthe reactor under a pressure of between approximately 5 bars andapproximately 160 bars.

In conventional manner, the reactor is fitted with two pipes 2 and 3:

an outlet pipe 3 to discharge a water saturated gaseous phase,essentially containing oxygen,

an outlet pipe 2 to discharge an essentially liquid phase chieflycontaining residual soluble organic matter and an essentially mineralsolid phase in suspension.

The injection of oxygen 6 is made by a sludge recirculation loop 7 frombase 8 of reactor 1 towards its upper part. This layout is advantageousbut not compulsory. It is also possible to inject oxygen into anotherpart of the reactor. A heat exchanger 9 is provided to recover andreturn the calories from treated effluents with a view for further use,for example, to preheat the effluent to be treated.

In accordance with the essential characteristic of the invention, aheterogeneous catalyst is placed in a basket container 4 above interface10 between the liquid phase and the gas phase present in the reactor insuch manner as to leave between said interface 10 and said catalyst asecurity volume which will prevent or at least minimise contact of saideffluent with said catalyst. This security volume is obtained bymaintaining sufficient partial pressure above the liquid whilemaintaining the latter at a given level. Means 11 made up of notches onthe inner wall of the reactor are provided to change the position ofsaid basket container. A heater 12 may be positioned proximate theheterogeneous catalyst's basket container 4 to heat the heterogeneouscatalyst, preferably to temperature substantially above the temperatureof the effluent.

Under the present example, the sludge is placed in this reactor under anoxygen/COR stoichiometry of 1.5, at a temperature of 235° C. and under atotal pressure of 38 bars. For comparison with tests without aheterogeneous catalyst, a heterogeneous catalyst in accordance with thepresent invention is placed in the autoclave. The catalyst used is acatalyst containing 0.5% platinum on an alumina support in the form ofcylindrical drops (2.8 mm×3.5 mm, type 73 catalyst produced by JohnsonMatthey) contained in a grid basket container placed horizontallyapproximately 30 cm above the liquid-gas interface at rest (nostirring).

Certain tests were carried out by adding to the sludge to be treated ahomogenous catalyst (copper sulphate with a copper content of 500 mg/l),a catalyst intended to accelerate COR reduction.

The results given in FIG. 1 show that the homogeneous copper catalystused alone (with no platinum based heterogeneous catalyst) onlyaccelerates the conversion kinetics of organic nitrogen (amino acids,peptides, proteins . . . ) into ammoniated nitrogen but does notcontribute to removing ammoniated nitrogen by oxidation compared with atest without copper. On the contrary, the 3 tests carried out in thepresence of the platinum catalyst show substantial reduction ofammoniated nitrogen of up to 86% after a reaction time of 1 hour.

It is observed from the results given in FIG. 2 that the presence of theplatinum catalyst does not in any way affect COR removal during the wetair oxidation reaction. Unlike the prior art, and in particular thedisclosures of EP patent 431 932, according to which the presence of aheterogeneous catalyst, for example containing platinum, in contact withthe effluent increases the removal rate of COR and ammoniated nitrogen,the use of the heterogeneous catalysts of the invention leads toextended nitrogen removal without affecting COR decline in any way.

It is therefore possible for example in the case of a residual watertreatment plant, by using wet air oxidation treatment, to remove totalammoniated nitrogen from sludge and to produce an effluent made upchiefly of volatile fatty acids, alcohols and ketones, said effluentforming a very efficient carbonated source to remove the nitrogencontained in the effluent entering the plant by biologicaldenitrification.

EXAMPLE 3

In a third series of tests, wet air oxidation of sludge from a treatmentplant is examined, the sludge having the following characteristics:

matter in suspension: 40.7 g/l

volatile matter: 60.7%

COR: 48.7 g/l

N-NNH4 content: 0.938 g/l

pH: 6.3

This sludge is placed in the reactor described in FIG. 4 in the presenceof an oxygen/COR stoichiometry of 1.5, at a temperature of 235° C. andunder a total pressure of 38 bars, with a reaction time of 15 min. Forcomparison with tests with no heterogeneous catalyst, the same load ofcatalyst containing 0.5% platinum as used for the second series of testsis placed in the autoclave in a grid basket container positioned eitherhorizontally approximately 30 cm above the liquid-gas interface at rest(test H3) or vertically approximately 80 cm above the liquid-gasinterface at rest (test V8). Certain tests are carried out by adding tothe sludge to be treated a homogeneous catalyst (copper sulphate, with acopper content of 500 mg/l)) a catalyst intended to accelerate CORdecrease. Optionally the initial pH of the sludge (6.3) is adjusted to avalue of 10 by adding a soda solution.

The results in Table 1 show that the increase of the initial pH of thesludge increases the catalytic oxidation efficacy of ammonia and thatthere is no significant formation of NOx nitrogen oxides, which wouldbecome soluble in the effluent treated in the form of NO₂ -- nitrite andNO₃ -- nitrate ions.

                  TABLE I                                                         ______________________________________                                                                 Contact                                                   time N-NH4 N-N02 N-NO3                                                     Catalysts Initial pH Final pH (min) (mg/l) (mg/l) (mg/l)                    ______________________________________                                        --     6.3      7.660    0     1407  12    n.d.                                 Pt(H3) 6.3 4.560 15 189 15 0.2                                                Pt(H3) 6.3 5.860 15 126 25 0.7                                                Cu 6.3 6.515 15 1361 4.5 n.d.                                                 Cu, Pt 6.3 6.115 15 867 9 0.4                                                 (V8)                                                                          Cu, Pt 10.0 6.815 15 696 8 0.3                                                (V8)                                                                        ______________________________________                                         n.d.: not determined                                                     

EXAMPLE 4

In a fourth series of tests wet air oxidation of an effluent derivedfrom a thermal sludge packaging process is examined which has thefollowing characteristics:

COR: 9.4 g/l

N-NH4 content: 1.52 g/l

pH: 7.85

This effluent is placed in an autoclave reactor in the presence of anoxygen/CIR stoichiometry of 1.5, at a temperature of 235° C. under atotal pressure of 35 bars with a reaction time of 15 min/ For comparisonwith a test with no heterogeneous catalyst, the same load of catalystcontaining 0.5% platinum already used for the second and third series oftests, is placed in the autoclave vertically approximately 80 cm abovethe liquid-gas interface at rest.

                  TABLE 2                                                         ______________________________________                                                                 Contact                                                  Final time N-NH4 N-N02 N-NO3                                                Catalysts Initial pH pH (min) (mg/l) (mg/l) (mg/l)                          ______________________________________                                        --      7.85     7.85    0     1521  323                                        Cu, Pt(H3) 7.85 6.7 15 720 117                                                Cu, Pt(H3) 10.0 7.6 15 600 116                                              ______________________________________                                    

The results given in Table 2 confirm that the increase of the initial pHof the effluent from 7.85 to 10.0 increases the efficacy of thecatalytic oxidation of ammonia and that there is no significant increasein the total oxidised forms of nitrogen, NO₂ nitrite and NO₃. nitrate inthe effluent treated.

EXAMPLE 5

FIG. 3 shows the effect of the final pH of the treated effluents on thepercentage of removal of ammoniated nitrogen in the different tests madein the presence of an oxygen/COR stoichiometry of 1.5, at a temperatureof 235° C. for 15 minutes under a total pressure of 38 bars in thepresence of a homogeneous catalyst (copper sulphate with a coppercontent of 500 mg/l) and a catalyst load containing 0.5% platinumcontained in a grid basket container placed either horizontallyapproximately 30 cm above the liquid-gas interface at rest (test H3) orvertically approximately 80 cm above the liquid gas interface at rest(V8). Optionally the initial pH of the effluent is adjusted to a valueof 10 by adding a soda solution.

These results confirm that the removal of ammoniated nitrogen is helpedby an increase in the effluent's pH;

EXAMPLE 6

In this test wet air oxidation of an effluent is examined which containsthe following compounds:

Urea (NH₂ CONH₂ : 0.026 mol/l)

Hexamethylenetetramine or HTM (C₆ H₁₂ N₄): 0.036 mol/l)

COR:7.6g/l

This effluent is placed in a reactor in the presence of an oxygen/CORstoichiometry of 1.5 at a temperature of 285° C. under a total pressureof 86 bars with a reaction time of 10 min. For comparison with a testwith no heterogeneous catalyst, a precious metal based catalyst isplaced in the autoclave on an alumina support in cylindrical honeycombshape comprising 0.5% platinum.

                  TABLE 3                                                         ______________________________________                                                       Treatment with                                                   Initial solution no catalyst Treatment with catalyst                        ______________________________________                                        COR g/l 7.6        0.4        0.03                                              N-NO3 g/l -- 0.008 0.075                                                      N-NO2 g/l -- 0 0.04                                                           N-NH4 g/l -- 2.76 0.045                                                       pH 7.5 9 6                                                                  ______________________________________                                    

The results obtained (see Table 3) show that in the presence of acatalyst, the percentage of ammonia removal reaches 98% and that thereis no significant increase in the total oxidised forms of nitrogen, NO₂₋nitrite and NO₃₋ nitrate, in the treated effluent.

EXAMPLE 7

In this test, wet air oxidation of an effluent is tested which containsthe following compounds:

Urea (NH₂ CONH₂): 0.0335 mol/l

Amino4-benzenesulfonamide (C₆ H₈ N₂ O₂ S): 0.0697 mol/l

COR: 11.4 g/l

pH: 6.8

This effluent is placed in an autoclave reactor in the presence of anoxygen/COR stoichiometry of 1.5, at a temperature of 285° C. under atotal pressure of 86 bars with a reaction time of 10 min. For comparisonwith a test with no heterogeneous catalyst, a catalyst containingprecious metals is placed in the autoclave on an alumina support incylindrical honeycomb form comprising 0.5% platinum.

                  TABLE 4                                                         ______________________________________                                                       Treatment with                                                   Initial solution no catalyst Treatment with catalyst                        ______________________________________                                        COR g/l 11.4       0.5        0.24                                              N-NO3 g/l -- 0.002 0.010                                                      N-NO2 g/l -- 0 0.010                                                          N-NH4 g/l -- 1.8 0.34                                                         pH  6.8 8.3 2.1                                                             ______________________________________                                    

The results obtained (Table 4) show that the presence of the catalystallows ammonia to be removed with a yield of 81% and that there is nosignificant increase in the total oxidised forms of nitrogen, NO₂₋nitrite and NO₃₋ nitrate, in the treated effluent.

EXAMPLE 7

In a third series of tests, a wet air oxidation reactor was used inaccordance with the present invention as shown in FIG. 6.

This reactor differs from the reactors shown in FIGS. 4 and 5essentially by the fact that it includes means 21 of recycling thegaseous phase drawn off in the upper part of the reactor. These meansalso comprise a hydroejector 22 placed at the exit of pump 20 ensuringrecirculation of the liquid phase (sludge) in the reactor through pipe7. This element allows the hot gaseous phase to be aspirated after itspassage through catalyst 4, to be mixed with the recirculated liquidphase and permits returning the mixture thus formed to the base of thereactor. Under the tests carried out with this reactor the aspirationflow of the gaseous phase with hydroejector 22 was set at 4 Nm3/h.

This reactor was tested on a sludge having the followingcharacteristics:

COR: 7.2 g/l

N-NH4 content: 1850 g/l

matter in suspension : 22.5 g/l

pH: 6.3

The treatment led to obtaining a treated effluent having an ammoniatednitrogen content of 963 mg/I, i.e. a reduction of 42%, and a COR contentof 887 mg/l, i.e. a reduction of 87%.

In comparison with treatment without a hydroejector, an improvement inCOR reduction was observed (85% instead of 68%).

Other tests were carried out with continuous adjustment of the pH inorder to maintain its value at approximately 7. This adjustment was madeby the addition of soda.

These tests were carried out on a thickened sludge with an initial pH of10 and a total nitrogen content of 1350 mg/l (NTK). They achieved a 80%reduction of COR and a 67% reduction of total nitrogen (NTK).

EXAMPLE 8

Other tests were also carried out with a reactor in accordance with FIG.7, differing from the reactor shown in FIG. 6 by the characteristicaccording to which the heterogeneous catalyst is not placed inside thereactor in a basket container 4 but in a cartridge 25 placed onrecycling loop 21 outside the reactor. A set of valves 26,27, 28 and adiversion 29 are provided so as to permit cartridge change withouthalting the operation of the reactor. Under normal operation, valves 26and 28 are closed and valve 27 is open to allow passage of the gaseousphase towards cartridge 25. If it is wished to change the catalyst,valve 27 is closed and valves 26 and 28 are opened enabling the gaseousphase to pass through diversion 29.

These tests led to achieving excellent COR and ammoniated nitrogenreduction.

All the results given above clearly show the numerous advantages relatedto the use of an effluent treatment according to the process of theinvention, in a reactor within which said effluents are subjected to wetair oxidation, in the presence of a heterogeneous catalyst andoptionally of a homogeneous catalyst and of at least one oxidising gassuch as air or oxygen at a temperature of between approximately 20° C.and approximately 350° C. under a total pressure of betweenapproximately 1 bar and approximately 160 bars. This is in no way arestrictive description of the invention in respect of the type ofeffluent, the formulation and conditions of use of the catalysts, nor ofthe conditions of use of the process representing the invention. Finallyit will be noted that the process described in the present patentapplication is compatible with the process of wet air oxidation withinternal recycling of solid residues described in French patentapplication n° 9403503 filed on Mar. 21st 1994 by the applicant.

We claim:
 1. A process of aqueous phase oxidation of effluentscomprising subjecting said effluents to oxidation in the presence of atleast one oxidizing agent inside a reactor in which a gaseous phase isset up above the liquid phase including the effluents, and subjectingsaid gaseous phase to catalysis in the presence of at least oneheterogeneous catalyst, positioned above the interface between theliquid phase and the gaseous phase and spaced inwardly of the walls ofthe reactor, said process being carried out at a temperature of betweenapproximately 20° C. and approximately 250° C. under a total pressure ofbetween approximately 1 and approximately 160 bars, in such manner as tomineralize at least part of the organic matter and total ammoniatednitrogen contained in said effluents and characterized in that itcomprises increasing the contact time between the gaseous phase and theheterogeneous catalyst by recycling at least part of said gaseous phasepresent in said oxidation reactor after its passage through saidheterogeneous catalyst by directing a portion of the gaseous phase fromthe oxidation reactor and then back into contact with said heterogeneouscatalyst in order to obtain substantial removal of NH₃, COR and volatileorganic compounds.
 2. The process of claim 1 wherein said oxidationprocess is carried out at a temperature lying between approximately 200°C. and 350° C. and under a total pressure of between approximately 20bars and approximately 160 bars.
 3. The process of claim 1 wherein saidheterogeneous catalyst includes two or more constituents selected fromthe group consisting of vanadium, niobium, chromium, molybdenum,tungsten, manganese, iron, cobalt, nickel, copper, cerium, platinum,rhodium, ruthenium, palladium, and iridium, and the mixtures andcompounds of one or more of these.
 4. The process of claim 1 whereinsaid heterogeneous catalyst is placed on a mineral support.
 5. Theprocess of claim 4 wherein said mineral support is chosen from the groupmade up of alumna, silica, zealots, titanium dioxide and zirconium. 6.The process of claim 4 wherein said heterogeneous catalyst is in theform of a metallic honeycomb.
 7. The process of claim 1 wherein saidoxidizing agent includes one or more compounds selected from the groupconsisting of air, oxygen enriched air, oxygen, ozone, hydrogenperoxide, peracids, gaseous chlorine, chlorine biocide, sodiumhypochlorite and potassium permanganate.
 8. The process of claim 1further including a gas moving within said reactor, said gas includingone or more compounds selected from the group consisting of air, oxygenenriched air, oxygen, ozone, water or nitrogen steam.
 9. The process ofclaim 1 wherein said aqueous phase oxidation is also carried out in thepresence of a homogeneous catalyst intended to increase the efficacy ofCOR reduction.
 10. The process of claim 10 wherein said homogeneouscatalyst is a metal selected from the group consisting of manganese,iron, cobalt, nickel, copper, zinc, and the mixtures and compounds ofone or more of them.
 11. The process of claim 10 wherein saidhomogeneous catalyst is a soluble compound of copper, zinc, or both. 12.The process of claim 11 wherein said homogeneous catalyst is coppersulfate.
 13. The process of claim 10 wherein the mass ratio of catalystto the chemical requirements of oxygen (COR) of the effluent beforetreatment lies between 0.0005 and 0.3.
 14. The process of claim 1further including a solid phase mixed in said liquid phase and furtherincluding recycling at least a portion of said solid phase through saidreactor.
 15. The process of claim 1 further including adjusting the pHof said effluents to a value of between 7 to 12 before said oxidation.16. The process of claim 1 further including heating the area of thereactor proximate said heterogeneous catalyst to a higher temperaturethan that of the effluent.
 17. The process of claim 1 further includingheating said heterogeneous catalyst to higher temperature than thetemperature of the effluents in said reactor.
 18. An oxidation reactorassembly for the treatment of effluents, comprising:a. an oxidationreactor having therein a gaseous phase disposed above a liquid phase,said liquid phase including the effluent to be treated; said reactorhaving a temperature of between approximately 20° C. and approximately350° C. and a pressure of between approximately 1 and approximately 160bars; b. an inlet connected to said reactor for receiving the effluentto be treated; c. a heterogeneous catalyst positioned above theinterface between the liquid phase and the gaseous phase and spacedinwardly of the walls of the reactor; d. an outlet connected to saidreactor for removing at least a portion of the gaseous phase aftercatalyzation; e. a gaseous phase recirculating circuit for recycling atleast a portion of the gaseous phase from the reactor and back intocontact with the heterogeneous catalyst so as to increase the contacttime between the gaseous phase and the heterogeneous catalyst, saidgaseous phase recycling circuit including a gaseous phase recycling paththat directs the gaseous phase from the reactor and back into thereactor in such a fashion that the gaseous phase is recycled intocontact the heterogeneous catalyst.
 19. The assembly of claim 18 furtherincluding means for recycling at least a portion of said liquid phaseback into said reactor.
 20. The assembly of claim 18 further including aholder for said heterogeneous catalyst wherein said holder is verticallyadjustable for supporting said heterogeneous catalyst above said liquidphase.
 21. The assembly of claim 18 further including a heater operativeto heat said heterogeneous catalyst above the temperature of saideffluent.
 22. The assembly of claim 21 wherein said heater is a heatingcollar disposed outside said reactor.
 23. The assembly of claim 18wherein at least a portion of said liquid phase is recycled back intosaid reactor and wherein at least a portion of said gaseous phase beingrecycled is mixed with said liquid phase being recycled.
 24. Theassembly of claim 23 further including a hydro-ejector and wherein saidmixing is via said hydro-ejector.
 25. The process of claim 1 wherein theheterogeneous catalyst is disposed above the liquid phase and positionedinteriorly of an exterior wall structure that forms a part of thereactor.